There have been major difficulties in the use of copper as an electrical connection material in microelectronic circuits. These difficulties relate to the reactive nature of copper. Copper which has been deposited on a substrate will often react with subsequently deposited silicon containing materials and when such reaction occurs, it may delaminate or blister. This is especially significant in the production of liquid crystal displays.
Flat panel liquid crystal displays have been under development for well over a decade. At the present time they are used in laptop computers and other applications where it is desirable to have displays of low volume, low weight and low power consumption. However, various technological difficulties have hampered the production of flat panel liquid crystal displays of any great size.
Liquid crystal displays include a large number of picture elements or pixels arranged in a rectangular array. For example, in a large area liquid crystal display having high resolution, a matrix may be composed of 1280 columns and 1024 rows of pixels. In a color display, each pixel may have three subpixels for the primary colors, and thus there may be a total of nearly four million subpixels. In active matrix liquid crystal displays, each subpixel must be controlled by an active element, preferably a thin film transistor (TFT), which is constructed on a glass substrate.
The thin film transistors must each in turn be controlled by appropriate electronic circuitry which drives the display. In active matrix displays each thin film transistor is connected to a gate control line (for a row of pixels) and a drain control line (for a column of pixels).
Liquid crystal displays used in such applications as portable televisions and laptop computers are generally illuminated by backlighting. A well-known problem is that only a small percentage (typically approximately three percent) of the light generated by the backlight gets through the liquid crystal display to the user. This is in part due to color filters associated with the pixels, but it is also due in large measure to the presence of the thin film transistors and the control lines extending from the edges of the panel to the thin film transistors. To the extent that the lines can be made more conductive, such as having gate lines made of highly conductive metals, the gate lines can be more narrow and a higher percentage of light may be transmitted through the liquid crystal panel.
As an alternative to using wider gate lines, which have the disadvantage noted above with respect to. light transmission, it is possible to use thicker gate lines. However, thicker gate lines significantly increase the probability of producing so-called "crossover defects" during subsequent processing. In this processing the TFT structure is fabricated over the gate line, and the increased thickness leads to shorts or discontinuities that adversely affect the structure and therefore the operation of the TFT.
Materials which have been used for gate lines include molybdenum, chromium, and a molybdenum-tantalum alloy. While some success has been achieved, these materials are not sufficiently conductive. The short gate line pulses that are provided by the display driver chips located on the periphery of the liquid crystal display are attenuated due to the resistance and changed in shape in travelling from the edge of the display to its interior and the edge of the display opposite the driver chip. This gate line pulse distortion results in non-uniformity of display brightness, reduction of gray scale display capability (i.e. lack of contrast in some areas and therefore lack of uniformity in contrast across the display) and often produces noticeable flicker.
Until the present time, it has not generally been possible to use the most conductive material, copper, to form the gate lines to the thin film transistors on a liquid crystal display panel. This is because copper is very reactive with the subsequent layers of silicon dioxide or silicon nitride that must be placed over the portions of the copper gate lines which act as the gates of the TFT's, In the case of silicon dioxide, delamination of the oxide film from the copper occurs. In the case of conventional silicon nitride the nitride film, and under certain conditions the copper, will blister. In addition, copper adhesion to glass substrates is often poor.
One solution to the copper adhesion problem is to increase the adhesion between copper and glass by using an adhesion laser such as chromium or titanium between the glass substrate and the copper line. However, this additional step increases cost, and does not address the main problem of reactivity and delamination when silicon dioxide or a conventional silicon nitride film is used over the copper to fabricate the gate insulator of the TFT.
One approach which directly addresses the copper reactivity problem is to deposit a copper line on the glass substrate and to encase or cap the copper line in another material, such as tantalum. Using this approach, a copper layer must be deposited and patterned using, for example, standard photoresist techniques. Then a layer of tantalum must be deposited and this layer must also be patterned. Those additional steps add considerable cost to the production process and increase the width of the gate line.
When an adhesion layer or a capping technique are used, the probability of crossover defects may also be increased due to the increased gate line thickness.
Thus, it would be highly advantageous to be able to form a liquid crystal display including thin film transistors having copper gate lines (and therefore copper gates) directly on a glass substrate without the need for an additional encapsulating protective metal such as Ta.